Algae, a collective term of marine algae, usually attaching to a seabed or some solid structures, is a single or a series of simple plant comprising of basic cells. Nutrients in algae include polysaccharides, proteins, lipids, pigments and low molecular weight substances. Proven by traditional Chinese medicine and modern scientific research, the main component of substances having activity of enhancing immunity and anti-cancer activity in algae is polysaccharides.
Algae includes red algae, green algae, brown algae, etc., and current research and application of algae polysaccharides mainly focus on brown algae polysaccharides. Brown algae is a higher class of algae, having about 250 genera and more than 1500 species. The body of brown algae is in yellow brown or dark brown and contains substances like polysaccharides, proteins, lipids, mannitol, etc. Some members of the class, such as japonica, also contain a lot of iodine in their cells.
Brown algae polysaccharides are important components of brown algae, including algin, fucoidan, laminaran, etc. Algin, usually referring to sodium alginate, is a linear copolymer with polysaccharide homopolymeric blocks of (1-4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues, which has a high content in brown algae. There are three types of segment chain structure thereof: M blocks of continuous M units, G blocks of continuous G units, and a MG block formed with alternately connected G and M units. Algin is also the most representative class of algae chemical products. Data shows that algin annual production in China is the highest in the world. For ease of storage and use, algin is generally converted into sodium alginate as a final product. As a sodium salt of poly anionic polysaccharide (alginic acid), sodium alginate has a very wide range of applications in industry for its inherent physical and chemical properties.
In food industry, sodium alginate is an excellent food additive, for its low calorie, nontoxicity, easy expansion, and high flexibility, and for when it is added to foods it plays many functions like coagulation, thickening, emulsifying, suspending, stabilizing, food drying prevention, etc.
In textile industry, sodium alginate has features like easy to color, high color yield, color bright and softening printed fabrics, etc., and is most commonly used paste in cotton fabrics reactive dye printing. Meanwhile, it can also be used as soluble fiber in industrial processes such as warp slurry, water proof processing, and lace manufacture.
In cosmetic industry, sodium alginate is used as toothpaste base, shampoos, hair setting agent, etc. In paper industry, it is used as sizing. In rubber industry, it can be used as latex concentrate, and also can be made as water-based coatings and water-resistant coating.
Sodium alginate is also an important biomedical material, being widely used as drug delivery agent, polymer film, cell encapsulation, wound dressings, surgical sponge, embolic agent, etc. It is attracting more and more attention in biomedical material science, clinical medicine, tissue engineering and pharmaceutical science and other fields. So far, algin production basically still uses alkaline digestion, the basic principle of which is to use sodium carbonate (Na2CO3) to convert various water-insoluble alginate salts into sodium alginate, to dissolve sodium alginate in water, and to obtain dry sodium alginate powder through filtration, acid/calcium precipitation, and sodium salt conversion.
Fucoidan is a heteropolysaccharide, comprising fucose and sulfate, as well as monosaccharides including galactose, xylose, mannose, uronic acid, etc. Due to its unique structure and excellent physiological activity, such as regulating blood lipids, lowing blood sugar, lowing blood pressure, anti-clotting, anti-tumor, anti-mutagenic and anti-radiation, anti-virus, enhancing immune function, etc., fucoidan has become one of hotspots of marine drug research of this century. Laminaran, is also a polysaccharide with a variety of physiological activities. The preparation of fucoidan and laminaran mainly utilizes wastes from the production of algin, iodine, and mannitol as main raw materials, which are subjected to grading-alcohol precipitation after water soaking.
Current algin production process has the following major disadvantages:
It has heavy water and energy consumption, and heavy pollution. This is also one of the key factors restricting the development of the algae industry. Alkaline digestion is simple in principle, but often requires dozens of processes in actual industrial production, some of which are very difficult. For example, in the concentrated sodium alginate slurry formed from japonica through the treatment of sodium carbonate, many water-insoluble cellulose and other ingredients need to be filtered. Due to high viscosity of the concentrated slurry, in addition to the addition of a filtering aid like diatomite, it also need consume a large amount of water for dilution, and has high requirements in quality of the water used. According to statistics, the production of one ton of finished sodium alginate needs about 700-1000 tons of water. Further, there are mainly two approaches i.e. acid addition and calcium salt addition in the chemical process of conventional alkaline digestion: first, water-insoluble alginate salts in brown algae are converted into water-soluble sodium alginate, adding acid or calcium ions makes sodium alginate form alginic acid or calcium alginate precipitates which after washing is converted back to sodium alginate, and finally sodium alginate is further processed into various products. Whichever method is used, it will produce a large amount of industrial waste water, which poses a serious threat to the ecological environment.
Meanwhile, current sodium alginate product is a single product variety, lack excellent quality, and have low added value. Sodium alginates in term of structure can be divided into three categories: high G/M ratio, medium G/M ratio, and low G/M ratio, in term of viscosity can be divided into ultra-low viscosity, low viscosity, medium viscosity, high viscosity, and ultra-high viscosity sodium alginate, in term of purity can be divided into three levels: industrial, food and medical. The current domestic production of sodium alginate are mostly medium-viscosity products. As health or pharmaceutical products, its application is limited due to its strong gelling property, low solubility, etc., and its activity cannot be fully achieved.
To overcome these difficulties, domestic and oversea researchers made long-term efforts on those processes targeting above disadvantages, and achieved a lot of important progress and accomplishments. These studies include improvement of the conventional process and development of a new process. For instance, a recently reported new process of algin reactive extrusion by foreign scholars has advantages like water-saving, time-saving, less alkaline use, etc., and the viscosity and the yield of the product get some degree of improvements, but the process cannot directly produce water-soluble polysaccharide with good biological activities. Modification to conventional algae polysaccharide products includes using biological, chemical or physical methods to degrade sodium alginate and fucoidan to alginate oligosaccharides, oligosaccharides, and low molecular weight fucose, to adjust sulfur content of fucoidan, etc.
Researches also confirmed that the activities of sodium alginate and fucoidan after degradation has been effectively improved, some low molecular weight sodium alginates already show heparin-like anti-tumor and anti-viral physiological activities and can be used for cardiovascular disease and virus medicinal research. Some of them have intestine adjustment and detoxification, lowering blood glucose and lipid, anti-clotting, anti-inflammatory, immunomodulatory effects and can be used as dietary food for diabetes, obesity, colorectal cancer, and habitual constipation patients.
Especially in recent years, unique physiological functions of modified low molecular weight algin or fucoidan continues to be discovered, and its activity and medicinal value has become one of the new research hotspots.
Further, polymannuronic acid (M) and polyguluronic acid (G) are unique components in algin molecule, and have not been found independently exist in nature. When the proportion of the two alduronic acid polysaccharides (M/G), or the structure and arrangement of such a block in algin varies, the performance of the algin will exhibit significantly difference. Therefore, there are many researchers obtain different polysaccharide fragments and oligosaccharide thereof with unique structures through different degradation and separation methods, as to study the unique biological activity thereof, and to develop drugs with special efficacy.
All these series of oligosaccharides or oligose with different structures greatly enrich the diversity of algae polysaccharide products. However, these studies all use algin of medium viscosity as raw materials, and does not fundamentally solve the problems in conventional production process of sodium alginates, like heavy water and energy consumption, heavy pollution, low yield/content, etc.
Microwave assisted rapid method for hydrolysis of sodium alginate for M/G ratio determination (Mahesh Chhatbar, Carbohydrate Polymers Vol 76 (2009) 650-656) discloses in a home-use microwave oven, using a oxalic acid solution or a dilute sulfuric acid solution as solvent partially degrades sodium alginate, and its main purpose is to find a simple, rapid, and mild method to determine the M/G (mannuronic acid/guluronic acid) ratio in sodium alginate. However, the sodium alginate material used in this method is still prepared by conventional extraction methods, and does not overcome heavy water and energy consumption and other defects in the conventional process.
Microwave assisted desulfation of sulfated polysaccharides (Diego A, Navaroo, Carbohydrate Polymers Vol 69 (2007) 742-747) discloses in a home use microwave oven, using a microwave-assisted method removes sulfur from red algae polysaccharide carrageenan, agar and fucoidan in brown algae, animal polysaccharides chondroitin sulfate, as to overcome the shortcomings of commonly used hydrochloric acid desulfurization method. The method still uses polysaccharides as starting material and has not overcome the defects like heavy water and energy consumption, etc. In addition, the purpose of pursuit of high desulfurization rate in the paper is intended to improve convenience of analyzing samples, and does not take into account the activity of polysaccharides.
Therefore, it is necessary to continue to carry out the research and development of new technology, new technology, new product in marine science and seaweed industry.